Which structure anchors the thin filaments to the midline and connects myofibrils to one another?

• A. Z disc
• B. M line
• C. SR
• D. A band

Answer: (A)

The Z disc is the structure that anchors the thin filaments and links neighboring myofibrils. It sits at the boundaries between adjacent sarcomeres and provides attachment points for actin filaments, holding them in place as the sarcomere contracts. Through desmin and other cytoskeletal connections, the Z discs help distribute forces across neighboring sarcomeres and stabilize the muscle fiber’s internal alignment. In contrast, the M line sits at the center of the sarcomere and anchors thick filaments, the sarcoplasmic reticulum stores calcium, and the A band is the region containing thick filaments and their overlap with thin filaments, not an anchor site for thin filaments to the midline.